The middle and lower reaches of the Yangtze River traverse a vast alluvial plain, vulnerable to erosion, resulting in frequent bank collapses, which could seriously affect the sustainable development of the local economy and society. From 2003 to 2022, a total of 1049 bank collapses occurred in the middle and lower reaches of the Yangtze River, spanning over 758.7 km in cumulative length. Although there has been a recent decline in bank collapses due to the ongoing riverbank protection projects, bank collapses still occur occasionally even in protected banks because of a complex array of influencing factors, including near-shore water-sediment dynamics, internal seepage processes within the riverbanks, alterations in soil properties, etc. Analysis results indicate that influencing factors contributing to bank collapses in the middle and lower reaches of the Yangtze River have significantly changed in recent years. Firstly, the continuous erosion of the river channel, particularly the near-shore bed leads to increased height differences between the riverbed and the banks thereby elevating potential for bank slope instability. Secondly, the adjustment of local river regimes causes shifts in flow rushing points and changes in extent of mainstream adjacency to the banks, which can lead to erosion and caving along inadequately safeguarded riverbanks. Thirdly, during post-flood water-fall periods when water levels rapidly decrease, infiltration pressure negatively impacts bank slope stability. Additionally, during pre-flood water-rise periods when flow velocity sharply increase, bank collapses are prone to occur due to intensified scouring caused by high-flow conditions.

In this study, the hydrodynamic characteristics of coastal freak waves are numerically investigated using an in-house code named a constraint interpolation profile (CIP)-based model. These freak waves are generated by a spatial-temporal focusing mechanism, where multiple water waves converge at a specific point in space and time to produce a large transient wave. The applicability of the current model in simulating the nearshore evolution of coastal freak waves is validated by comparing the simulated results with previous experimental data. The hydrodynamic characteristics of coastal freak waves under varying coastal topography, wave parameters and the focusing positions of freak wave group are comprehensively analyzed. Empirical formulas for quickly predicting the breaking and run-up of coastal freak waves are summarized. Additionally, the differences between coastal freak waves and other wave types, including regular, irregular and solitary waves are compared. It is found that the nearshore run-up of freak waves is smaller than that of solitary waves (with a relative ratio range of 0.33-0.5) but larger than that of irregular and regular waves (with relative ratio ranges of about 1.25 and 3.3, respectively). All the results indicate that solitary waves can be employed to evaluate the disaster characteristics of coastal freak waves, which provides valuable references for engineering disaster warning and prevention.

The 1D-2D coupled hydrodynamic models have been widely developed, to simulate free-surface flows in large shallow water systems. A deeply coupled solution of 1D and 2D submodels refers to the involvement of a solution of flow equations across the 1D-2D interfaces, which will ensure the ability, stability and accuracy of the 1D-2D coupled model in simulating various flow regimes and scenarios. A deeply coupled solution is relatively easy for explicit submodels, but difficult for implicit submodels because of their complex iterative solutions. Techniques for coupling the solution of implicit 1D and 2D submodels are systematically studied, which includes a dimension reduction pretreatment for linking 1D and 2D grids, an upwind scaling method for solving the flow advection across a 1D-2D interface, a method for the drying-wetting simulation across 1D-2D interfaces, etc. The domain-decomposition prediction-correction method is extended to solve velocity–pressure coupling problems of the mixed 1D and 2D subdomains synchronously. Based on these techniques, a 1D-2D deeply coupled model is developed, which allows large time steps, requires only minor data exchange at 1D-2D interfaces, and can be well parallelized. Model test is first done using a real dam-break flow and a hypothetical periodic flow in an experimental flume, to demonstrate the ability of the new model in simulating unsteady supercritical and subcritical flows. The simulation results are shown to agree with the experimental measurements, the simulation results of an existing 1D-2D coupled model and the HEC-RAS. The second test uses a real large river–lake system, and the efficiency of the proposed model is compared with those of a pure 1D model, a pure 2D model and a newly reported 1D-2D coupled model (the proposed model is shown to run about 2 orders of magnitude faster than the newly reported model). Moreover, four 1D-2D coupled models, characterized by different levels of coupling degrees, are tested and compared. With the key physical terms of the governing equations being solved across the 1D-2D interfaces, the proposed model achieves essential improvements in accuracy compared to the loosely coupled model.

2023年6月9-15日,国际大坝委员会(ICOLD)第91届年会在瑞典哥德堡举行.期间先后召开了各专业委员会专题研讨会、专业委员会会议、技术展览、"大坝安全管理"国际研讨会及国际大坝委员会第91届执行会议等,来自78个国家和地区的1238名代表参加了会议.

为提升堤坝溃决险情处置和溃决灾害防御能力,多年来长江科学院河流研究所采用物理模型试验、水槽试验、理论分析、数值模拟等方法研究了堤坝漫溢溃决的机理、模型与模拟技术.其主要成果包括:揭示了堤坝漫溢溃决机理,解析了"溯源陡坎冲刷"在堤坝溃决过程中的作用,提出了溯源陡坎冲刷模式和堤坝漫溢溃决模式;基于物理机制,研发了溯源陡坎冲刷二维数学模型和堤坝漫溢溃决数学模型;发展了适应溃坝水流急变特征的一、二、三维溃坝洪水运动模拟技术及地形处理方法,并初步探索了溃坝水流的三维流场与动压特性;总结评述了相关领域的研究进展.研究成果成功应用于唐家山、白格等历次堰塞湖溃决险情的应急处置和决策制定,并为今后堤坝(含堰塞坝)溃决险情的科学应对提供了技术参考和经验借鉴.

The construction and operation of the Three Gorges Reservoir and the cascade reservoirs upstream have significantly altered the processes of flow and sediment in the main and tributary channels of the Yangtze River. This has led to substantial adjustments in the riverbed through erosion and deposition, thereby impacting flood protection, water resource utilization, navigation, and the aquatic environment in the Yangtze River basin. In this study, prototype measurements were used to analyze the variations in runoff and sediment load in the main channel of the Yangtze River, as well as the changes and evolution of the riverbed. Mathematical modeling was done to predict the trends in reservoir sedimentation and riverbed adjustments. The results indicate that, apart from the significantly increased runoff and sediment load in the river source region (Zhimenda station) over the past two decades, there is no clear unidirectional trend of increasing or decreasing in the main and tributary annual runoff of the Yangtze River. However, the release of reservoir outflows undergoes significant changes throughout the year due to reservoir regulation. Suspended sediment load in the upper Yangtze River has been decreasing since the 1990s, especially after the operation of the Three Gorges Reservoir and the four cascade reservoirs in the lower reach of the Jinsha River. The factors influencing flow and sediment variations include mainly climate change and human activities such as reservoir operation and soil and water conservation. The significant changes in flow and sediment conditions have disrupted the original relative equilibrium state of the main channel of the Yangtze River, leading to riverbed adjustments. The river sections in the upper reach, located within reservoir areas, have shifted from erosional state under natural conditions to accumulative state, while the middle and lower reaches have transitioned from a relatively equilibrium state to a process dominated by erosion and reconstruction. Overall, the river regime in the reservoir area and downstream of the Three Gorges Dam remains relatively stable, but there have been adjustments in some local river sections, for example, in the curved sections, particularly in the sharply curved sections downstream of the Three Gorges Dam, where gradual or abrupt chute cutoff has occurred. In the foreseeable future, the river channel downstream of the Three Gorges Dam will remain in an unsaturated state regarding sediment transport. The process of river channel erosion will persist for a long time and have far-reaching consequences. Some long straight sections, multiple bifurcation sections, and sections with large curvature are expected to undergo certain adjustments in the river regime, necessitating continuous observation, long-term monitoring, and timely river management and channel governance.

The relationship between the Jingjiang River and the Dongting Lake involves complex water-sediment exchange. Changes in water-sediment conditions have significant impacts on fluvial processes, flood safety, and water resources utilization in the middle Changjiang River. Since the operation of the Three Gorges Project, the water-sediment conditions have changed greatly, and the relationship between the Jingjiang River and Dongting Lake has been adjusted. This paper reviews the research progress on the following points: a) the water-sediment exchange relationship, b) the erosion and silting evolution of the Jingjiang River, especially the river diversion gates, c) the erosion and silting evolution at the three distributary flood channels and the Dongting Lake, and d) the water level changes at Chenglingji. The study concludes that the diversion flow of the three diversion outlets of Jingjiang River has slightly reduced, and the amount of sediment diversion has greatly reduced. The Ouchi River has seen sharp adjustment, resulting in deteriorated inflow conditions to Dongting Lake. Dongting Lake has experienced a change from siltation to micro-scouring, leading to a drop in lake area’s water level. Additionally, the Chenglingji-Hankou reach has been scoured, resulting in water level drawdown at Chenglingji. However, deficiencies still exist in the research of driving mechanism, and disputes remain over the changes in Jingjiang flood levels and the future evolution trend of Dongting Lake. Quantitative analysis and systematic and complete analysis are in lack. Issues that need to be addressed in future research are also proposed.

三峡工程运用后,"清水"下泄,导致长江与洞庭湖发生冲淤调整,对坝下游江湖防洪与水资源利用等影响已逐步显现,而上游干支流控制性水利水电工程陆续建成投运,与三峡工程共同作用后对坝下游的影响将更加深远.在以往研究的基础上,结合新近实测资料进一步研究长江与洞庭湖水沙冲淤变化,分析江湖冲淤变化对防洪与水资源利用的影响,并提出相应的对策建议,可为未来江湖治理思路、治理方案的制定与实施等提供参考借鉴.

The adjustment of scouring and silting downstream of reservoirs is a long-term complex process, which has a certain impact on flood protection, navigation and ecology. To explore the evolution trend of Shashi-Hankou reach in the middle reach of the Yangtze River and provide a reference for the prevention and control of adverse changes, the temporal and spatial changes of deposition and erosion of grouped suspended sediment after the construction of the Three Gorges Dam (TGD) are analyzed. After the construction of TGD, the sediments of each particle size group in Shashi-Jianli reach are eroded; and the 0.125-0.250 mm particle size group has the largest amount of sediment erosion when the discharge at Shashi station is 10 000-25 000 m3/s. Taking 0.125 mm as the boundary particle size of coarse and fine particles for the Jianli-Luoshan reach, the reach is characterized by "coarse deposition" in flood season and storage period, and "fine erosion" in low water season and pre-flood falling period. The sediments of each particle size group in the Luoshan-Hankou reach are eroded, and the larger the discharge, the greater the amount of erosion. On this basis, by establishing the relationship between discharge and sediment of different particle size groups and analyzing the sediment carrying capacity formula, it is found that after the construction TGD, the scouring and silting of the Shashi-Jianli reach and the Luoshan-Hankou reach are affected jointly by sediment and hydraulic factors, and the impact of sediment factors on the Shashi-Jianli reach is greater than that on the Luoshan-Hankou reach. At the same time, the "coarse deposition and fine erosion" in the Jianli-Luoshan reach is mainly affected by the recovery degree of sediment concentration, which is related to the scour replenishment of the upstream riverbed and the exchange of coarse and fine sediments in this reach, reflecting that the sediment carrying capacity of the river channel under scour condition can be increased by grading adjustment.

Treatment of the free-surface pressure boundary (FSPB) remains a challenge for the three-dimensional (3D) nonhydrostatic hydrodynamic models using a vertical staggered-grid variable arrangement. Through the tests of the small-amplitude wave, it was found that the pressure of the top layer of the vertical grid is semihydrostatic for vertical staggered-grid nonhydrostatic models. Based on this finding, a modified ghost-cell FSPB treatment is proposed and a fully nonhydrostatic staggered-grid model developed. The new method is simple and easy to implement, and has the self-calibration property of its parameters. The model was further tested using a laboratory experiment of nonlinear waves propagating over a bar toward a beach. Sensitivity studies of model parameters and grid scales were performed. The vertical staggered-grid model using the new FSPB treatment is shown to require only coarse vertical grids to achieve accurate simulations of nonhydrostatic free-surface flows.

针对天然河道崩岸现象,采用概化模型试验和数值计算相结合的方法,初步分析了不同水位变化速率对河道崩岸的影响.研究结果表明:水位变化对崩岸有明显影响,尤其在水位降落阶段,水位降落速率越大,崩岸越明显.岸坡稳定性计算结果显示涨水时稳定系数随水位上升而增大,至高水位后稳定系数先减小后保持稳定,水位降落阶段稳定系数减小,且与降落速率密切相关.研究成果可为分析不同水位变化速率下岸坡稳定性的演化特征并采取相应的河道整治措施提供理论依据.

2020年汛期洞庭湖区堤岸共发生险情41处,均得到及时处置和有效控制.基于2020年汛期江湖汇流段水文过程、洞庭湖堤岸险情时空分布,南鼎垸浪拔湖镇东洲段散浸,屈原垸东大堤、白泥湖垸新建八组管涌,屈原垸西大堤浪损等险情处置过程等资料,从水文、地质等方面探讨了湖区堤岸险情的主要影响因素.结果表明:2020年汛期七里山站高水位持续时间明显长于2008～2019年同时段,日最大水位涨幅为0.37 m,日最大流量涨幅为5700 m3/s.洞庭湖区堤岸发生管涌险情最为频繁,占65.9％.累计险情数量与七里山站高水位持续时间密切相关.湖区堤岸险情还与洪水期风浪、地质条件、险工险段分布及穿堤建筑物隐患有关.研究成果对今后洞庭湖区堤防建设及管理有较好的指导意义.

长江中下游地处冲积平原,洪水灾害较为频繁,沿江堤防是生命财产和生产设施的第一道屏障.其中荆江河段洪水峰高量大,且两岸支流众多、江湖关系复杂,防洪形势尤为严峻.以上荆江松滋江堤防洪保护区为研究对象,分析了可能产生的堤防溃口;在对复杂的荆江-洞庭湖水系进行概化的基础上,建立一二维耦合数学模型,研究了1000 a一遇洪水条件下松滋江堤发生溃决后长江干流及防洪保护区内的洪水演进过程,并以最大淹没水深、淹没时间、最大流速、受影响人口及洪水损失为风险要素,分析了保护区内的洪水风险分布情况.文章共分为2篇,此为第二篇,旨在阐述保护区内洪水演进过程及风险分布特征.结果表明,涴市横堤发生溃决后造成的洪水损失最为严重,在实际防汛工作中需要重点关注.

基于河道崩岸现有研究成果,对崩岸类型的划分进行了梳理,进而将河道崩岸的影响因素概括为水流动力条件、河道边界条件,及其他因素3大类,并从数值模拟和概化模型试验2个方面总结了河道崩岸研究的进展.在此基础上,分析讨论了当前河道崩岸机理研究中存在的不足,如未能全面考虑各种因素影响、崩岸数值模拟技术不够完善、概化模型试验精度需进一步提高等.最后,对未来崩岸研究工作的开展,提出了需关注的重点与建议.

2020年汛期长江流域持续出现了5次洪水,荆江三口分洪能刀及洞庭湖出流变化直接关系到长江中游防洪安全.基于此,根据最新实测资料分析了2020年汛期荆江三口分洪能力及洞庭湖出流变化规律,并取得了以下主要认识:①与2003～2018年期间相比,2020年7,8月份荆江三口分流量大幅增加,且在枝城站同等大流量下荆江三口分流量有一定程度增大,其中松滋口分流量明显增大,藕池口略有增大,太平口则有所减小.②荆江三口分流比与枝城站流量呈正相关关系,2020年汛期枝城流量增至38 000 m3/s以上,三口分流比基本在20％以上,其中7月25日三口分流比最大,其数值为26.1％,分流量高达11 300 m3/s.③与2003年相比,2020年汛期在螺山站同等大流量下水位出现不同程度抬高的迹象,可能与该站下游区间入汇流量明显偏大等因素有关;6月13日之前,城陵矶站与螺山站流量比值变化主要受到洞庭湖四水来流的影响;6月14日至9月25日,其比值变化主要受长江干流流量变化的影响,监利站流量快速增加或减小,其比值呈现相反变化规律;监利站流量缓慢变化,其比值变化规律不明显.

Flood events caused by the overtopping failure of earth-rock dams often lead to catastrophic disasters. Therefore, examining the cause of the dam failure and studying the subsequent flood evolution processes are extremely important for disaster prevention and mitigation. This work reviewed the phenomena and mechanism of the overtopping failure of earth-rock dams and recent researches using physical experiments and numerical simulations. The scale design of physical model test is discussed, and the research findings of dam break experiments under different scales, dam types, and experimental conditions are summarized. Research progresses of numerical simulations of dam breaks using parametric models, simplified physically-based models, and detailed physically-based models are reviewed. Advances in numerical and experimental studies of the evolution of flood caused by dam failure are highlighted. After reviewing the literature, some future research directions are proposed. They include examining the influence of river boundary conditions on dam failure and the subsequent flood propagation processes, reconciling the mechanism of the failure of heterogeneous earth-rock dams, investigating the erosion law of sediment-laden flow during dam breaks, studying the downstream scouring and silting and riverbed sediment sorting after dam failure, and evaluating the impact of dam failure on the aquatic ecosystems.

絮凝沉降性能是河湖疏浚淤泥资源化处理中重要且未完善的研究课题.以武汉沙湖、官桥湖及南湖3种淤泥为研究对象,通过沉降筒试验,利用清浑交界面沉降速度和上清液浊度研究了粒径分布、初始含沙量及高分子聚合物对河湖淤泥絮凝沉降特性的影响规律,并探讨了沉降筒尺寸对试验结果的影响.试验结果表明:河湖淤泥粒径越小,淤泥絮凝沉降越慢,上清液浊度越小;随初始含沙量的增加,河湖淤泥整体沉降速度变小,但当初始含沙量增加到一定值后,初始含沙量的影响作用开始变弱;高分子聚合物会促进河湖淤泥絮凝沉降;沉降筒尺寸虽然对试验结果略有影响,但对于分析河湖淤泥絮凝沉降特性无较大影响.研究河湖淤泥絮凝沉降特性对于河湖通航、蓄洪、水质修复、疏浚淤泥的处理均有重要意义.

石首弯道段位于长江中游下荆江首端,河道形态复杂多变,河床演变较为剧烈.在大量河道治理及航道整治工程的控制下,岸坡稳定性及航道条件得到了增强和改善.三峡水库蓄水以来,坝下荆江河段的来水来沙条件发生了显著变化,石首弯道段也出现了大幅冲刷,但在一系列河道治理及航道整治工程的作用下,除了局部河段深泓摆动和冲淤变化较为明显外,总体河势基本稳定.随着三峡水库及上游干支流水库群不断建成和运用,该河段河势仍将持续调整.近岸河床的不断冲刷下切,已有工程难以适应新形势下的河床变形,不断出现崩塌冲失,对现有堤防、河势控制工程、护岸工程和航道整治工程等的安全运行以及涉水工程与航运等带来新的问题.提出应抓紧深入研究三峡水库及上游控制性水库运用初期长江中下游重点河段河势变化与综合治理方案等重大技术问题.

A vertical two-dimensional (2D) nonhydrostatic numerical model for dam-break flows is proposed. This model combines a depth-averaged shallow-water equations (SWEs) model and a nonhydrostatic core using mode splitting. The nonhydrostatic core is solved under the free surface provided by the SWE model, in which tracking the complex and discontinuous free surface of dam-break flows in vertical spaces is avoided. In a test of an ideal dam-break flow, the new model was revealed able to capture the discontinuous free surface of dam-break flows and produce reasonable simulation results for velocity and nonhydrostatic pressure. The proposed model was further tested using an experiment of dam-break flow over dry bed with a trapezoidal step. The new model with the first-order Riemann solver produces a discontinuous reflected wavefront in the simulated free-surface profiles. The simulated free-surface profiles are improved by the new model with the second-order Riemann solver, and further improvements by using the volume-of-fluid (VOF) three-dimensional (3D) model are slight. (C) 2018 American Society of Civil Engineers.

长江中下游地处冲积平原,洪水灾害较为频繁,沿江堤防是生命财产和生产设施的第一道屏障.其中荆江河段洪水峰高量大,且两岸支流众多、江湖关系复杂,防洪形势尤为严峻.以上荆江松滋江堤防洪保护区为研究对象,分析了可能产生的堤防溃口;在对复杂的荆江一洞庭湖水系进行概化的基础上,建立一二维耦合数学模型,研究了1 000 a一遇洪水条件下松滋江堤发生溃决后长江干流及防洪保护区内的洪水演进过程,并以最大淹没水深、淹没时间及最大流速为风险要素,分析了保护区内的洪水风险分布情况.文章共分为2篇,此为第一篇,旨在阐述一二维耦合数学模型建立与率定验证过程.